Measuring a voltage without voltage drop and preventing polarity change

Thread Starter

InterestingStuff

Joined Nov 6, 2019
4
Hi everyone,

I have looked through the forum and I couldn't find a solution.

I will start my explaining the situation:

I want to measure the voltage from a potentiometer. This potentiometer is coupled to a motor shaft, and indicates the position of a motor. By measuring the voltage I can convert it to the motor position. The voltage range goes from 0.8 to 9V and the motor has 50 different positions, which means roughly every 165mV represents a different position. See picture attached for a visual representation of the system. I do not have access / can't change anything within the blue box.

Now the problem, and why I am looking for help:

I am measuring the voltage through a DAQ. This DAQ needs to be connected to a computer to properly define which pins will be set as inputs and outputs. When the DAQ is disconnected from the computer, the pins configuration are undefined, and the pins which I use to read the voltage can have a voltage from 0 to 2V. This voltage will be applied to the potentiometer and to the motor, which will start moving. This can damage the supplier's instrument. I need to prevent the DAQ from applying a voltage/current to the motor.

My initial idea was to add a diode between the potentiometer and the DAQ. However, since a diode has a non-linear V x I curve, the equation used to convert voltage into position becomes unreliable, since the voltage drop through the diode would change as the current increases. Using another device to measure the voltage is not an option, I joined the project recently and this is at the final stage, we need to do it with the DAQ available. I am mostly looking for an analog solution.

Any help will be greatly appreciated, what are the alternatives?

P.S: In most cases the DAQ will always be connected to the computer and this won't be a major issue, however, if we have problems with the PC power (i.e, power failures, anything that makes the computer restart or whatever) we have the possibility of having a big problem.

P.S2: Sorry if this is the wrong forum, I was unsure if I should post here or on measurement. Since this a measurement related problem, but the solution is an electronic circuit/component, I decided to post here instead. Apologies in advance if it was wrong!
 

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Dodgydave

Joined Jun 22, 2012
8,477
Use an OP amp buffer from the potentiometer to the Daq, the potetiometer has a rotary range of 0 to 270deg, with 0.9 to 9V, that's 30mV per degree rotation.
 
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